Chromium in the form of Chromate (Cr04-) is a widely employed and highly effective corrosion preventive pigment. It is the de facto standard for corrosion prevention in primers applied to aluminum, particularly aerospace aluminum stock (2024-T3, and certain 7000 series alloys, for instance). In addition, hexavalant chromate in the form of strontium-chromate is the benchmark for corrosion prevention in coil coating primers (whether based on Epoxy-Melamine, Polyester or other binder systems) when applied to Zinc, Zinc-Aluminum or similar galvanized or electro-galvanized coil stock.
There has been extensive activity to eliminate hexavalent chromium from these primer systems, which efforts are driven by two related impeti. First, hexavelent chromium is toxic, thus efforts to identify a performance-equal substitute to strontium chromate are easily understood. Secondly, there are directives promoted in Europe that require a limit on chromium content, such that a manufactured article may more readily be recycled.
For the foregoing reasons there have been numerous academic research projects1 and an untold number of industry product development exercises devoted to finding a replacement for strontium-chromate. There is also a substantial patent literature history describing some of these efforts by industry. 1Progress in Organic Coatings 47 (2003) 174-182—Buchheit et al.
In almost every case examined however, there appears to be one or more flaws in the touted benefits of the inventive compounds. Typically, the claimed chromate-free inhibitor is in fact not demonstrated in a coating system, but rather in a primer alone. This may disguise a performance flaw, for example, that the inhibitive pigment leads to blistering. The blistering may result either from traces of water-soluble salts that are the by-products of the pigment synthesis, or from the intrinsically high solubility of the touted composition itself.
Sometimes, the touted composition is offered based on data for a coating system that is applied only to ferrous metal surfaces. Though it may be free of chromium, the anti-corrosive pigment provides insufficient electrochemical over-potential to prevent corrosion of highly reactive metals such as aluminum or zinc. Finally, it is often the case that the benefit of a chrome-free anti-corrosive pigment is made on the basis of unrealistically short accelerated testing; which is deemed unreliable by those skilled in the art. Indeed, many patented offerings, when independently tested, fail to deliver the long duration of outstanding corrosion prevention expected of strontium chromate in benchmark accelerated tests.
The present invention overcomes these deficiencies in previous chrome-free anti-corrosive pigments. It provides that the inventive compositions are able to deliver performance on a par with strontium chromate when evaluated directly alongside the same in realistic coating systems, appropriate for the substrate and end-use application. The present invention in one preferred form does use a ratio of elements in its composition that is somewhat similar (though clearly different) to ones previously disclosed, though in an entirely difference field of application, as a catalyst2. It differs from the prior disclosed systems in terms of the ratios of elements used to make an amorphous solid solution of zirconium and vanadium oxides or hydroxides. The approximate range of composition included in this invention is between Zr2 V3O12.nH2O and Zr3V4O16.nH2O, where n is between 0 and 6. 2Journal of Catalysis: 177, 343-351, (1998); Khodakov et al.
The present invention affords finished compositions within the above ranges that are, by virtue of the synthetic method employed, essentially free of soluble salts. Furthermore, due to the lack of tracer salt contaminants, their resistance to blistering in typical coating systems is greatly improved.
The use of vanadium containing compounds as a component of an anti-corrosive surface treatment on reactive metals, such as 2024-T3 aluminum is not in and of itself novel. In fact, a prior application by Buchheit et al describes the use of vanadate in a chromate-free conversion coating. These are different from the present inventive compositions in that they are not used as pigments in the primer formula, but rather provide a solid base on the aluminum that will improve adhesion of a subsequently applied anti-corrosive primer and any topcoat.3 3U.S. Pat. No. 7,135,075—Corrosion resistant coating with self-healing characteristics, Buchheit et al, issued Nov. 14, 2006.
Vandium salts or compounds in a primer coating formula also have been disclosed in prior patent applications, such as that to Hager et al in U.S. Pat. No. 6,077,885. The disclosure in the '885 patent is distinct from that in the present application on the following grounds. The '885 patent explicitly describes the use of relatively soluble alkali metal vanadate salts such as sodium metavanadate, with or without the addition of other compounds such as cerium acetate or cerium oxalate as direct additions to a primer formula. Though they claim good blistering resistance after accelerated salt fog testing, it should be noted that claim is made for a primer alone, in which the likelihood of blistering is greatly reduced. Moreover, no comparative performance against a positive control in which Strontium Chromate is the anti-corrosive pigment is provided. Thus, the actual benefit in use of the primers made in accordance with the '885 patent over the standard commercial product cannot be assessed.4 4U.S. Pat. No. 6,077,885—Chromate-free protective coatings, Hager et al, issued Jun. 20, 2000.
Zirconium compounds have also been employed in anti-corrosive primers in substitution for chromate anti-corrosive pigments. One example is U.S. Pat. No. 6,716,370, which discloses zirconium containing supra-molecular oxo-anion compounds which may be used as a component of a primer. These materials do not however contain any vanadate or vanadium containing ions. The compositions disclosed in U.S. Pat. No. 6,716,370 are based upon complex molybdenum and/or tungsten and/or phosphorus compounds with silicon and are thus clearly different from the present invention.5 5U.S. Pat. No. 6,716,370—Supramolecular oxo-anion corrosion inhibitors, Kendig, issued Apr. 6, 2004.
Various organic compounds, based either on complex carboxylic acids or organo-phosphonic acids and theyr salts with zirconium, bismuth, alkali earths and the like are disclosed in U.S. Pat. No. 6,403,826, or references therein, assigned to Ciba Geigy. As was the case with the earlier cited Kendig patent, these are clearly different in nature from the zirconium vanadium compositions disclosed herein.6 6U.S. Pat. No. 6,403,826—Corrosion-inhibiting coating composition for metals, Braig et al, issued Jun. 11, 2002.